Travelling wave tubes (TWTs) amplify the RF oscillations in the GHz frequency range by the interaction of a beam of charged particles, usually electrons, and an electromagnetic wave which propagates with suitable arrangements so as to amplify the electromagnetic wave.
TWTs are particularly attractive and convenient because through their adoption, high amplification with low RF noise at relatively high powers and over extremely wide bands (of the order of hundreds of MHz) can be obtained.
As a consequence these electronic tubes find wide application in radar equipment, microwave telecommunication systems, satellites, etc.
In use, these TWTs have the undesirable feature that they allow possible generation of spurious oscillations at the edges of their operating bandwidth and, particularly, close to the upper limit of their bandwidth.
A further inconvenience connected with these oscillation instability phenomena is that the anode voltage must be controlled accurately so that particular interaction modes, which would otherwise give way to these spurious oscillations are not generated.
This required that, as an example, any powder modulation be achieved only through the control grid.
Many attempts have been made to reduce the impact of these inconveniences:
(1) Bandwidth limitation: the problem of band edge oscillation is avoided, rather than cured, by reducing the TWT bandwidth to ensure synchronism between phase velocity of the circuit wave and the beam velocity close to interdiction frequencies (Italian Pat. No. 676,571). PA0 (2) Use of dissipating media which intervene also in the tube operating bandwidth. Oscillations are prevented by introducing attenuation (Power TWTs--J. F. Gitting-American Elseviers Publishing Co.--N.Y. 1965). PA0 (3) Use of low Q resonators coupled to interconnected cells or cavities.
As for point (1) there are power limitation and precarious stability, which impose strict control over anode voltage.
As for point (2) there is gain reduction due to losses introduced, acting also within the operational bandwidth.
As for point (3) the operating principles (use of resonating elements at single frequencies) require that the band of possible range of oscillation is covered by numerous elements, each tuned with great accuracy and therefore with great waste of work, bearing in mind that materials must be procured and tested to great accuracy with respect to dielectric characteristics as regards permittivity. Consequently, earlier attempts along these lines aimed at solving of the problem of spurious fringe oscillations consisted in the introduction, within the tube structure, of elements resonating at the undesired frequencies, associated with dissipating elements, with the aim of attenuating the gain of the tube at these spurious frequencies.
From an industrial viewpoint, the preparation of resonating elements is expensive because fine accuracy resonating frequency tuning of these elements is required.